Modification Mechanism of Ce-Treatment on Inclusions in High-Al Ferritic Steel

Rare earth element treatment is one of the most promising methods to solve problems in the metallurgical process of high-Al steel. 6.3 pct Al ferritic steel was prepared and treated with 0.04 pct Ce under laboratory conditions. The inclusions in the steel before and after Ce-treatment were observed by SEM-EDS. After Ce addition, CeS and Ce₂O₂S inclusions were produced in the steel. The modification mechanism of Ce on inclusions was studied by thermodynamic analysis and mismatch calculation. The Al₂O₃ inclusions were modified into CeS–CeAlO₃–Al₂O₃ and CeS–CeAlO₃, in which the CeAlO₃ is generated by the reaction of Al₂O₃ and dissolved Ce, and the CeS precipitates on the surface of Al₂O₃ and CeAlO₃ by heterogeneous nucleation. The AlN inclusions were modified into single-particle and aggregated CeS–AlN by heterogeneous precipitation on the surface of CeS. Furthermore, the aggregation of AlN was analyzed based on the cavity bridge theory, and the relationship between the critical aggregation distance and the particle radius of AlN is \({d}_{\text{C, AlN}}\text{ = 0.134}{\left({r}_{{\text{P}}_{1}}{r}_{{\text{P}}_{2}}\right)}^{0.5}\text{ - 0.02}\left({r}_{{\text{P}}_{1}}\text{ + }{r}_{{\text{P}}_{2}}\right)\) $d_{\text{C, AlN}}\text{= 0.134}{\left(r_{{\text{P}}_1},r_{{\text{P}}_2}\right)}^{0.5}\text{- 0.02}\left(r_{{\text{P}}_1},,\text{+},,r_{{\text{P}}_2}\right)$ . The promoting effects of CeS on the AlN aggregated process are as follows: I. increasing the amount of nucleation point and precipitation temperature of AlN; II. increasing the initial equivalent radius of AlN particles; and III. CeS–AlN acts as the core during multiple aggregation processes with AlN.